The present invention is directed to an electronic device comprising a quantum dot and an organic host, a mixture comprising a quantum dot and an organic host, a quantum dot, a method for preparing a quantum dot (QD), and a formulation including the mixture or the quantum dot.
Organic light emitting diodes (OLEDs) as promising next generation display technology have drawn considerable attention since more than two decades, because OLED materials are versatility in synthesis, rich in color, light in weight, and less power consumption. The performance of OLEDs, especially those based on evaporated small molecules, has shown excellent performance, like color, lifetime and driving voltage, allowing OLEDs to enter commercial applications.
In OLED devices, electrons and holes are injected from opposite electrodes and recombine to form excitons, either singlet or triplet. Radiative decay from singlet excitons is fast (fluorescence), but that from triplet excitons (phosphorescence) is inhibited by the requirement of spin conservation and is therefore often inefficient. According to quantum statistics, three triplets per singlets are formed in the OLED if the probability of exciton formation is not spin-dependent. The probability of singlet exciton formation for devices based on fluorescent materials is thus only 25%. Hence, a fundamental limit of an internal quantum efficiency of 25% is put on OLEDs which are solely based on singlet emitters. This limit can be overcome by incorporating phosphorescent dopants, also called triplet emitters, usually complexes containing a heavy metal, which can increase spin-orbital coupling and harvest both singlet and triplet excitons. However, the metal complex is difficult to synthesize and it has stability problems. So far, a stable (deep) blue triplet emitter is still elusive. Moreover, because the triplet level of the organic materials is typically at least 0.5 eV higher than singlet level, a blue triplet emitter having itself a big band-gap (or HOMO-LUMO gap) will put extremely hard requirements on host materials and the charge transport materials in the adjacent layers.
On the other hand, another class of emissive material, so-called quantum dot or mono-dispersive nanocrystal as described below, has also drawn considerable attention in the last years. The advantages of quantum dot are: 1) theoretical internal quantum efficiency as high as 100%, compared to 25% of the singlet organic emitter; 2) soluble in common organic solvents; 3) emission wavelength can be easily tuned by the core size; 4) narrow emission spectrum; 5) intrinsic stability in inorganic materials.
The first mono-dispersive nanocrystals including a semiconducting material, also referred to herein as quantum dots or QDs, were based on CdE (E=S, Se, Te) and were produced using the so called TOPO (trioctyl phosphine oxide) method by Bawendi and later modified by Katari et al. A review on synthesis of QDs is given by Murray, Norris and Bawendi, “Synthesis and characterization of nearly monodisperse CdE (E=sulfur, selen, tellurium) semiconductor nanocrystallites”, J. Am. Chem. Soc. 115[19], 8706-8715, 1993. The mostly-reported caps of quantum dots are based on trioctylphosphine oxide (TOPO) or trioctylphosphine (TOP), which are supposed to have electron transporting properties.
The first light-emitting diode based on CdSe QDs was reported by Alivisatos et al., “Light emitting diodes made from cadmium selenide nanocrystals and a semiconducting polymer”, Nature (London) 370[6488], 354-357, 1994, where a multilayer consisting of QDs was sandwiched between PPV (poly(p-phenylene-vinylene)) and an electrode, giving emission in red upon applying voltage. Bulovic et al., “Electroluminescence from single monolayers of nanocrystals in molecular organic devices. Nature (London) 420[6917], 800-803, 2002 describe use of a single monolayer of CdSe QDs sandwiched between two organic layers.
But one major problem of known QD LEDs is the huge energy level offset between QDs and adjacent organic layers, for example CdSe QDs have a HOMO of −6.6 eV and LUMO of −4.4 eV (WO 2003/084292, WO 2007/095173), and on the other side functional organic materials have usually a LUMO >−3.0 eV and a HOMO >−5.6 eV. The big energy offset prevents that QDs are efficiently electronically active in electroluminescent devices or other electronic devices.
Therefore the object of the present invention is to provide an improved electronic device.
This can be realized by an electronic device according to claim 1, a mixture according to claim 9, and a quantum dot according to claim 15 of the present invention. The present invention further relates to a formulation including the mixture or the quantum dot, an application of the mixture or the quantum dot, a method for preparing the quantum dot, and the electronic device including the mixture or the quantum dot.
According to a first embodiment an electronic device includes a cathode, an anode, an emissive layer, a hole injection or charge generation layer deposited between the anode and the emissive layer, wherein the emissive layer includes at least one quantum dot and at least one host material.
In a variation of the first embodiment, an electronic device is provided, including a cathode, an anode, an emissive or active layer including at least one quantum dot and one organic host, and a charge generation layer deposited between the anode and the emissive layer.
Preferably the charge generation layer has a working function or electron affinity higher than 5.6 eV.
Typically, as mentioned before, the QDs available so far have a HOMO lower than −6.0 eV. However the standard transparent anode ITO (Indium Tin Oxide) used in OLED or QD-LED, has a working function of approximately −5.0 eV, and standard functional organic materials, especially hole injection materials (HIMs), have a HOMO>−5.6 eV. The huge offset in energy levels makes the hole injection from anode into QDs very difficult.
The present invention solves this problem by using a charge generation layer, which preferably has a work function or electron affinity higher than 5.6 eV, very preferably higher than 5.8 eV, and particularly preferably higher than 6.0 eV.
Orbital energies such as HOMO or LUMO as discussed within the present invention relate to distances from the vacuum level. If the distance of a molecular orbital energy level from the vacuum level is high, the orbital is said to be low or deep. In contrast, if the distance of a molecular orbital energy level is low, the orbital is said to be high.
Orbital energies (HOMO/LUMO) and band gaps can easily be determined according to methods well known in the art. These orbital levels may be measured by photoemission, e.g. X-ray photoelectron spectroscopy (XPS) and Ultra-violet photoelectron spectroscopy (UPS) or by cyclovoltammetry (CV) for oxidation and reduction, for instance. It is well understood in the field that the absolute energy levels are dependent of the method used, and even if the evaluation method for the same method, for example the onset point and peak point on the CV curved results in different values. Therefore, a reasonable comparison should be made by the same evaluation method of the same measurement method. More recently, quantum chemical calculations, for example Density Functional Theory (DFT), have become well-established methods to calculate the molecular orbitals, particularly occupied molecular orbitals. The method used herein in order to determine orbital energies and band gaps is outlined in Example 1. Preferably, the hole injection layer or charge generation layer is transparent, e.g. transparent for outcoupling light or incident light.
A hole injection layer or charge generation layer suitable for embodiments described herein can be selected from a charge generation layer or an intermediate connector layer for tandem OLEDs, as disclosed for example in U.S. Pat. No. 7,564,182 and US 2006/0240277A1.
According to one embodiment, in the electronic device the hole injection layer or charge generation layer is selected from or comprises one or more transition metal oxides. In a further embodiment, the hole injection layer is preferably selected from or comprise vanadium oxide (VOx), molybdenum oxide (MoOx), ruthenium oxide (RuOx) and tungsten oxide (WOx).